There is no need to change his name since according to Islam a child's name has no effects on his/her behavior. There are no verses in the Quran or authentic hadeeth that say baby names have any effect on personality. If anyone says otherwise, they are speaking without knowledge.
For this reason you can keep the name Rohaan and look for other methods to improve the child's behavior. Many children are very naughty when they are small but they slowly improve as they grow up.
Islamically you are allowed to change the child's name if you would like. The name Ahad is an acceptable name, but we do not believe that changing the name will have an effect on the child's behavior.
New question from the same person:
And if I want to change his name so can i?
Can u tell me is ahad good name
according to his star in Islamic way?
Yes, you can change his name. Islam allows parents to change a child's name if they want to. Ahad is a good name since it has a good meaning. In Islam we do not depend on stars, other astrological methods or numerology when choosing names. All of these practices are against Islam and are considered a type of shirk (believing that there is another god besides Allah). All power is with Allah and all good fortune comes from Allah, not from stars or numbers; therefore if a name has a good meaning, then it is a lucky name since it is pleasing to Allah. Ahad is also one of the names of Allah, similar to Ali, for this reason it may please Allah to use such a name insha'Allah.